1. Field of the Invention
This invention relates to a solar reflecting mirror, e.g. a parabolic shaped solar reflecting glass mirror and a method of making same and more particularly, to a solar reflecting mirror made of shaped mirror segments.
2. Description of the Available Technology
At the present time, there is interest to increase the efficiency of solar collectors, e.g. and not limiting to the discussion, improve the efficiency of solar mirrors, e.g. parabolic shaped mirrors, used to reflect the sun's rays to a device located at the focal point of the parabolic mirror. The device is usually of the type known in the art to convert the sun's energy to another form of useable energy, e.g. electric energy and/or heat. In another embodiment of the prior art, the parabolic mirror is a primary mirror reflecting the sun's rays to a secondary mirror positioned relative to the focal point of the primary mirror to reflect the sun's rays to the converting device.
In general, the parabolic shaped mirror includes a parabolic shaped substrate having a reflective surface, e.g. a silver coating on the convex surface of the shaped substrate. The preferred material of the shaped substrate is soda-lime-silica glass because of the high yield in shaping a flat glass sheet to a parabolic sheet or substrate; the low cost of making flat glass sheets, and the high yield and low cost of applying a solar reflective coating on a surface of the shaped glass substrate.
Although soda-lime-silica glass is an acceptable material for the substrate for solar reflecting mirrors, there are limitations to the use of glass. More particularly, in the shaping process, a flat glass sheet is heated to temperatures above 1200° Fahrenheit (hereinafter also referred to as “F”) and shaped into the parabolic shape. During the heating and shaping of the glass sheet, the alkali ions, e.g. the sodium ions in the glass sheet diffuse, or leech, out of the glass sheet. Further, during exposure of the parabolic shaped glass substrate to solar energy, e.g. long-term environmental exposure, additional sodium ions leech out of the glass substrate. As is appreciated by those skilled in the art, the leeching or diffusion of the sodium ions from the glass is an expected occurrence, and at low temperatures is a slow process. However, heating the glass and/or the long term environmental exposure of the glass to solar energy accelerates the leeching or diffusion of sodium ions out of the glass, and increases the amount of sodium ions that leech out of the glass. The sodium ions leeching out of the glass react with moisture in the atmosphere, and convert from sodium ions to sodium compounds, e.g. sodium hydroxide and sodium carbonate. The sodium compounds can etch the surface of the glass and can deposit as a precipitate on the surface of the glass. The sodium compound precipitates decrease the transmission of visible light through the glass, e.g. in the case of the parabolic shaped glass substrate, decrease transmission of solar energy to the reflective coating on the convex surface of the shaped glass substrate, and decrease the transmission of the solar energy reflected from the reflecting coating through the shaped glass substrate to the concave surface of the shaped glass substrate.
Further as is appreciated by those skilled in the art, the surface of the shaped glass substrates is a specular surface, and the solar energy is incident on the concave surface of the glass substrate as parallel light rays. The parallel light rays are reflected from the concave surface, and reflected from the reflective coating, as convergent light rays. The sodium compound precipitate on the concave glass surfaces converts the specular surface to a non-specular or diffusing surface directing the light rays reflected from, and passing through, the precipitate away from the focal point of the primary mirror. The term “specular surface” as used herein means a light reflective surface where a light ray incident on the reflective surface has an angle of incidence equal to the angle of reflection. The term “non-specular or diffusing surface” as used herein means a reflective surface where a light ray incident on the reflective surface has an angle of incidence different from the angle of reflection.
Present techniques to remove and/or to eliminate the sodium compound precipitate from the concave surface of a parabolic mirror include cleaning the surfaces and/or enclosing the concave surface of the mirror to provide a sealed chamber having an inert gas to prevent the sodium ion from forming the precipitate. Present techniques for removing scratches include buffing the surfaces of the glass sheet having the scratches. All of these techniques to ensure the surfaces of the solar mirror remain a specular surface are expensive.
Barrier layers are known in the art, e.g. disclosed in U.S. Pat. Nos. 4,238,276; 5,270,615; 5,830,252 and 6,027,766, and U.S. patent application Ser. No. 08/597,543; U.S. patent application Ser. No. 12/709,045 filed even date in the name of Abhinav Bhandari et al and titled SOLAR REFLECTING MIRROR HAVING A PROTECTIVE COATING AND METHOD OF MAKING SAME, and U.S. Publication 2007/0275253A1. One of the limitations of the presently available alkali barrier layers and/or scratch resistant layers is that they are efficient for use on flat or shaped surfaces of glass substrates, but are not efficient for use on a flat surface that is subsequently shaped to a curved surface, e.g. a concave surface of a parabolic mirror. There is little, if any, recognition or discussion in the prior art of the problems that have to be solved when a substrate coated with a barrier layer and/or a scratch resistant layer is shaped from a flat-coated substrate to a parabolic shaped coated substrate. More particularly, there is little, if any, discussion in the prior art of eliminating the cracks in, and/or the buckling of, the coating as the contour of the coated glass is changed from a glass piece having flat surface to a shaped glass substrate having a concave surface. As is recognized by the instant application, when the barrier coating is stressed, the coating cracks and the sodium ions are exposed to the atmosphere and form the sodium compound precipitate on the surfaces of the glass substrate, and/or when the barrier coating and/or the scratch resistant coating buckles the surface changes from a specular surface to a non-specular or diffusing surface.
As can now be appreciated by those skilled in the art, it would be advantages to provide a solar reflecting mirror and method of making a solar reflecting mirror that does not change, or does minimizes the change of, the reflecting surface from a specular surface to a non-specular or diffusing surface.